1. Field of the Invention
The present invention relates to a turbocharger including a compressor housing which is provided with a return passage for returning part of compressed air to an upstream side.
2. Description of the Related Art
There has been known a turbocharger including a bearing housing configured to rotatably hold a turbine shaft that is provided with a turbine wheel (turbine impeller) on one end and a compressor wheel (compressor impeller) provided on the other end. The turbocharger is connected to an engine and the turbine wheel is rotated by exhaust gas discharged from the engine. The rotation of the turbine wheel rotates the compressor wheel via the turbine shaft.
The turbocharger compresses air with the rotation of the compressor wheel and sends the compressed air to the engine. In a vehicle or the like equipped with a turbocharger, when throttle valves in the engine are closed for deceleration, a boost pressure rises whereas a flow rate of air drops, so that the turbocharger may cause a surge and noise. To address this, as disclosed in Japanese Patent Application Laid-Open Publication No. 05-163954, there has been a widely used configuration of a compressor housing provided with a return passage making upstream and downstream sides of a compressor wheel communicate with each other, the return passage configured to be opened and closed by an air bypass valve. The employment of the configuration makes it possible to avoid a surge by opening the air bypass valve and returning part of the compressed air to the upstream side of the compressor wheel when the boost pressure rises.